Electrical filter



Oct. 28, 1941. T. B. D. TERRONI ELECTRICAL FILTER 2 Sheets-Sheet 1 Filed Nov. 20, 1940 Z Z m a m n R m R W W 72 :L O H E B v Z Z Y 1 3 PM. n

%Kg 6 z KsH 9 l T. 5D. TERRONI M-M W MOM /S Oct. 28, 1941. T TERRONI 2,260,494

ELECTRICAL FILTER Filed Nov. 20, 1940 2 Sheets-Sheet 2 [rare/aim T. BD. TI I R RONI Attorney 5 Patented Oct. 28, 1941 new UNITEDTQSTATES PATENT-OFFICE I .-.2,260;494 l g ELECTRICAL F LT Teseo Bruno Dante Terroni; Broadgreen, Liver-- pool, England, assignor to. Automatic Tele-.

phone & Electric Company, Limited, London, England, a. British company The present invention is concerned with improvements in or relating to wave filters and has for its object the provision of a wave filtering arrangement which has a constant or substantially constant resistance for its input impedance over a range of frequencies extending from to infinity cycles per second."

According to one feature of the invention a constant or substantially constant resistance wavefilter comprises a unit consisting of a resistance connected in series with a filter network having a termination equal or substantially equal to said resistance, said unit being connected in parallel with a second unit consisting of a resistance in s'eries'with another filter network terminated in a resistance, the two filter networks viewed from the sides to which the series resistances areconnected having iterative impedances the square root of the product'of which is equal or substantially equal to'each of said resistances.

According toa second feature of the invention a constant or substantially constant resistance wave filter comprises a unit consisting of a resistance connected in parallel with a filter network having a termination equal or substantially equal to said resistance'connected in series with a resistance in parallel with a filter-network terminated in a resistance,v thetwo filter networks viewed from the side to which the parallel resistances are connected having iterative impedances the square root of the product of which is equal or substantially equal to each of said resistances.

The expression filter network as used in this elments connected in known manner to have any desired characteristics and low attenuation over one range of frequencies and high attenuationat other frequencies. Usually such a filter network includes a plurality of recurrent sectors or sections suitably related to have filter characteristics but conveniently one of the filter networks employed in accordance with the invention may consist of a single section or half section which While not quite so efficient is much less expensive and will give sufficiently satisfactory results for most purposes. If still further economy is required and the efiiciency is not so important both filter networks may consist of a single section or half section.

The invention will be better understood from the following description of a number of methods of carrying it into effect, reference being had to the accompanying drawings comprising Figs. 1-10.

In the drawings .jmid shunt iterative impedances ZKsH andthis is tet and under these conditions and therefore Z= lproduct of two inverse impedances Referring now to Fig. 2 this shows a wellknown single T structure which is terminated by its mid series iterative impedances ZKS and this is given by I Z ZKg= /Z,Z [l+ The TI structure of Fig. '3' is terminated by its given byr In a filter structure such as a low pass filter, a section of which is shown in Fig, 4, Z1 is approximately equal to a'wLo and Z2 is approximately equal to (neglecting the small pure resistance components thereof), so that g I /Z Z /%Z (for a low pass filter) These results will sun obtain'in the case of a high pass filter andit will thus be seen that the constant resistance property is retained.

One filter will serve in effect as a balance fil-.

ter, and it will be appreciated that a substantially constant input resistance may: be obtained by utilising for the balance filter one half or more sections of the proper filter it is desired t utilise.

loss which would normally be taken up by the amplifier. t

The filter shown in Fig. 9 is preferable to that shown in Fig. 8 as it utilises less components in the balancing filter. A further example of a substantially constant resistance filter utilising a minimum number of componentsin the balancing filter is shown in Fig. 10.

It will be seen 'from this figure that the balancacteristic at the terminals l4 and 15, to that at terminals I5 and I6. and in addition the half,

' section balancing filter is matched to the termi- Reference should now be had to Fig. 6 which shows another type of constant, resistance struc.- ture to which the invention can be applied and in which Z=R providing Z1Z2=R.

If, as shown in Fig. 7 Z2 is replaced by a mid-' series terminated filter having, an iterative impedance ZKs'a'nd Z1 is replaced by a-balance filter having a mid-shunt iterative impedance ZKSH 80 arranged that .ZKsXZKsH=. R, then the input impedance at the terminals [0 and I'Lwill be equal to R over the complete range. of frequencies from 0 to infinity. The same remarks will, ofcourse, apply it Z1 be replaced by 'ZKs and One practical example of the arrangement of Fig. 6 is-shown in Fig. 8 where- Z2 is replaced. by a shunt-derived mid-series terminated filter which is, designed to give the required pass and attenuation characteristics. Leads l2 and I3 will extend via-further sections of the filter to a 600 ohms termination. Zris-replaced by a balance arrangement which may comprise on half or more sections of the filter replacing Z2, the section shown being of the mid-shunt terminated series-derived type and having a 600- .ohms termination. The measured input impedance curve for this arrangement ispractically pureresistance with small humps near the cut-ofi frequency. 1

It will beappreciated that by increasing the number of sections in; the mid-shunt terminated balance filter a better constancy of input resistance can be obtained, but the arrangement becomes correspondingly more expensive.

As an alternative, Zzin Fig. 6 could be replaced by a shunt derived (.m section). mid-shunt terminated'filter andZi' by one half or. more equivalent sections of the former filter whose first section is the mid-series termination of a series-derived m type filter. Fig, 9 shows an arrangement ofthis' type. which utilises only one half section of balancing filter and arranged to be terminated on its far side so as to give a good matching impedance .with the 600 ohms termination.

I .In both cases theattenuation of the-'filt'er in the pass range is increased by about 6zde'cibels nating resistance as closely as possible by employing a mid-shunt derived m type filter having an m of approximately 0.6.

Similarly, impedance matching at the terminals EF is maintained as closely as possible by employing a mid-shunt half section of a mid-series derived filter.

It will be appreciated that-the principles describedare not limited to one type of filter and they apply equally well to low pass, .highpass, band pass and band elimination filters.

Iclaims 1. A .wave filter having asubstantially constant resistance over a range-of frequencies extending from 0 to infinity cycles per second comprising a unit consisting of a resi-stanceconnectedin series with a filter network having a termination equal or substantially equalto said resistance said unit being connected in parallel with asecond unit consisting of a further resistance in-series with another filter network terminated in a resistance th .two filter networks .viewed' from the sides to which the series resistances are connected having iterative impedances the square root -.ofthe-.prod not of which is equal. or substantially, equal to each of said resistances, characterised inthat the filter network of one of the units consists of a single section or half-section.

2. A-wave filter having a constant or substan- V second, comprising a unit consisting :of a resist ance connected in parallelwith a filter network havinga termination equal or substantially equal to said resistancesaid unit being connected in' series-with a second unitconsisting of a resistance. connected in -parallel. with -;a filter network terminated in a're'sistan ce, the two filter networks viewedfrom the sides to'whichtheparallel resistances are connected; having; iterative imped ances the square rootof the productof which is equal. or substantially equal to each; of said resistances characterised in that the filter network of one of the units consists of a singl section or half section;v t v I I 3. A wavefilter asclaimed'in claim 1 in which both' filter networks consist of a single section or half section.

4. A wave filterasiclaimed in claim 2 in which both filternetworks consist of a single section or. half section... I j

} TEsEOBRUNO DANTE TERRQNI. 

